How to know when batt. is dead

[nikokapo]

k
i always tried measuring a normal (AA, AAA or 9V) battery with a multimeter
and it always says the voltage the battery indicates (+/- 0.1 or so sometimes)

but the real thing is, can i know WHEN a battery is dead?

the materials i have are: 2 AAA and one 9v, a green LED and a multimeter

can someone explain to me how to measure it?

 

[charleytown55il]

with alkalines, 1.2 volts is what i consider to be about used up.
with ni-mh, 1.05 volts it about used up and 1.20 - 1.25 volts is fully charged.
with a 9 volt, just apply this: it has 6 small 1.5 volt cells in it so 6 or so volts is dead.

btw, while im thinking about it, if you are ever using a 12 volt lead acid battery, 12.6-12.8 is fully charged and below 12.0 volts is dead.

 

[nikokapo]

but it is weird as the batteries always show the same reading, i mean. if the battery's capacity is of 2v, then the reading will say 2v!!

that's why i cant know whenever they are dead

 

[Benm]

You should measure the voltage under load, not with only a multimeter. Alkalines will read 1.5 volt open circuit from beginning to near end, no way to tell if a battery is at 90 or 10% that way.

A good measuring load is one that draws at least 1/10th the capacity, something like 200 mA for a AA(A) cell should do.

With NiMH's, voltage is hardly an indication of charge status at all.

 

[Curiously_Coherent]

Hi nikokapo,

I'm really no expert on Multimeters, but that's weird that you're not seeing a voltage drop as your batteries lose power. Are your leads plugged in the COM and V[ch937]mA ports? And is your selector dial set to the 20V (not 2V) setting? This seems to work for me. (I use this great little Multimeter I bought from DX: http://www.dealextreme.com/details.dx/sku.594 The backlight alone is worth the price.)

I mean no offence if you're an experienced electronics tech or something... just tryin' to be helpful.

CC

 

[charleytown55il]

if you are only measuring ONE 1.5 volt battery, then use the 2 volt setting for more accuracy.

 

[nikokapo]

Hi nikokapo,

I'm really no expert on Multimeters, but that's weird that you're not seeing a voltage drop as your batteries lose power. Are your leads plugged in the COM and V[ch937]mA ports? And is your selector dial set to the 20V (not 2V) setting?  This seems to work for me.  (I use this great little Multimeter I bought from DX: http://www.dealextreme.com/details.dx/sku.594  The backlight alone is worth the price.)

I mean no offence if you're an experienced electronics tech or something... just tryin' to be helpful.

CC

dont worry i take NO offence, thank you but i am measuring them correctly

and i have to say i was WRONG with what i said before

despite not using a load to measure batteries, one of them (the 9v) DID lose capacity, the multimeter read 1.56v (i repeat, from a 9v battery ). one of the 1.5v measured 1.26v.

so i was wrong, but thank you everybody anyway .

i will try to use some ultrabright leds to measure current and see if it drops too fast.

 

[equlizer]

When i check my batteries full load (4.5v) it shows it as 4.52v but when i turn off the laser, it shows it as 4.1v :-?